In the past, to carry out spectrophotometric determination on samples that are not transparent to light, the following ATR (Attenuated Total Reflectance) measurement was performed: the totally reflecting surface of a crystal, which has a high index of refraction, is attached closely to a sample, and light is irradiated on the ATR crystal at an incident angle greater than the critical angle (total reflection angle), so that the light is totally reflected, and an evanescent wave generated on the surface of the ATR crystal infiltrates into the sample; in that way, the properties of the sample are measured. When carrying out the ATR measurement, if the measured position of the sample can't be ascertained by observation, the specific position where the ATR measurement is performed will be unknown. Therefore, a mechanism with which the measured position of the sample can be observed and set is required.
Among ATR measurement techniques, the techniques for observing and setting the measured positions of a sample, as described in U.S. Pat. No. 5,093,580, issued on Mar. 3, 1992, to Donald W. Sting, particularly at column 3, line 43, to column 4, line 31, and at column 7, line 63 to column 8, line 29, with reference being made to FIG. 1; and as described in Japanese Patent Publication No. JP1994-34529, at paragraphs 0010-0011, with reference being made to FIG. 1, are known.
The following technique is described in U.S. Pat. No. 5,093,580: a visible light source (2) and a radiant energy source (3) can be switched by switching a switch mirror (6), to irradiate the sample with visible light (4) or radiant energy light (8), and the visual image can be confirmed through a viewing port (13), or an ATR measurement can be carried out with a detector (25). In the technique described in the patent document 1, the following structure is formed: visible light (4) and radiant energy light (8) are irradiated through a common light path and common optical systems (20, 21) to an ATR crystal (22) and a sample (26).
The following technique is described in Japanese Patent Publication No. JP1994-34528: illumination light (8) for ATR measurement is irradiated on an ATR crystal (1) at an incident angle greater than the critical angle, the reflected light is reflected by a Cassegrain objective optical system (2) and is detected by an optical detector (5), meanwhile illumination light (7) for observation is irradiated at an incident angle that is smaller than the critical angle and meets regular reflection criteria, and the reflected light is reflected by a Cassegrain objective optical system (2) and detected by an ocular lens (6).
In the past ATR measurements, to ascertain the measured positions of a sample, a camera for shooting pictures must be provided at the same side as infrared light irradiation; however, when shooting with the camera, a visible light source that can emit an appropriate quantity of light is required (for illumination). Here, under case that the sample and the ATR crystal are transparent to visible light, the camera can be used for shooting, provided that an illumination light source is arranged at the fixture side where the sample is pressed against the ATR crystal. However, under case that the sample is non-transparent to visible light, such as the structures described in U.S. Pat. No. 5,093,580 and JP1994-34528, illumination is required at the camera side.
However, in the technique described in U.S. Pat. No. 5,093,580, since the visible light and the ATR measuring light pass through a common light path, the visible light (illumination light) will also be totally reflected, resulting in the following problem: there is nearly no light scattered from the sample, only illumination light that contains nearly no sample information comes to the observer side (e.g., the camera), and nearly no image of the sample can be observed in the observed image.
In the technique described in JP1994-34528, since the illumination light (7) for observation irradiates at an angle that meets the regular reflection criteria, the light that is regularly reflected from the surface of the ATR crystal forms strong background light and superposes the observed image; therefore, it is difficult to judge in the image.
In view of the above problems, the technical task of the present invention is to provide a technical solution, with which the measured positions of a sample in an ATR measurement can be observed clearly.